The present invention relates to compensating for distortion of a received waveform. The distortion may be manifest in intersymbol interference and the concomitant increase in bit error rate (“BER”) thereby reducing the performance of the communication system. More specifically, the invention relates to a novel system and method of compensating for phase distortion of a received waveform containing digital symbols at one of a predetermined number of phase angles. One of the received waveforms the present invention is directed towards is a continuous phase modulation (“CPM”) waveform received from a satellite transponder.
Satellite transponders typically are comprised of a series of filters and hard-limiting amplifiers which modify the phase angle and amplitude of each symbol of the CPM phase constellation. Continuous phase modulation (“CPM”) waveforms undergo appreciable distortion due to the effects of the bandlimiting and hard-limiting satellite transponder. A typical CPM waveform used for satellite communications is the MIL-STD-181B (“181B”) waveform. Other well-known waveforms and some proprietary waveforms also use CPM and are contemplated with the use of the present invention. The 181B CPM constellation contains four possible frequencies per symbol which are multiples of alternating frequency deviations. Other waveforms may use 4-, 8-, and 16-ary constellations which take advantage of channel conditions to get the highest possible data throughput. Regardless of the constellation used, CPM symbols transition in phase per unit time (frequency). At a receiver the frequency errors appear, at that instant, to change the phase constellation. The distortion on each phase is induced by a number of elements, the largest of which is the satellite transponder. This distortion has its largest effect on the 16-ary CPM waveform and may prevent a receiver from synchronizing with a transponder on the satellite. The distortion also increases the BER of data being transmitted by the waveform. The BER may be used as a metric to indicate the performance of a transmission system. The lower the BER, the higher the performance and data throughput since less time must be spent retransmitting data.
A typical prior art solution to the distortion problem is to develop an inverse filter which predistorts the waveform prior to transmission. This solution requires modification to the transmitter itself. This type of solution may be useful for transmitters now being deployed as well as for transmitters that are easily accessed and modified but is highly impractical for existing satellite transponders. Another typical prior art solution uses an inverse filter to remove the distortion from the received waveform at the receiver. In this case, a post-distortion Least Mean Square Decision Feedback Equalizer (“LMSDFE”) method is typically chosen. In this application, the traditional LMSDFE equalizer can remove some of the intersymbol interference at low symbol rates, but at higher symbol rates the modem performance is far off from the same bit error rate achieved in an environment which only contains additive white gaussian noise (“AWGN”).
Observation of satellite communication channels with bandlimiting filters and hard-limiting amplifiers has shown that waveforms, notably the CPM waveform, is predictably distorted in phase during symbol transitions. Large phase transitions incur a larger, proportional amount of distortion when compared to smaller phase transitions. Additionally, the amount of distortion is independent of the angular location of any particular symbol. A characterization of phase error as a function of all the possible clockwise and counterclockwise adjacent phase transitions for the CPM phase constellation illuminates this proportional property of the distortion caused by the intersymbol interference. Once this error is characterized for all the symbols, the error can be removed by modifying the received phase angle for a symbol. One way to modify the received phase angle of a symbol is by subtracting the error value from the received angle. Another way to modify the received phase angle of a symbol is to multiply, via complex multiplication, the received phase angle by a derived error factor that is a function of either the reverse or forward phase shift or is a function of both the reverse and forward phase shifts.
The present invention avoids the problems of the prior art by exploiting the predictable phase distortion during symbol transitions discussed above. One embodiment of the present invention compensates for distortion of a received waveform by measuring the reverse and forward phase shifts for a given symbol then modifies the phase angle for the given symbol as a function of both the reverse and forward phase shifts to thereby compensate for phase distortion of the received waveform. This method has been shown to outperform the traditional LMSDFE equalizer especially when considering the noise enhancement inherent in LMSDFE equalizers.
Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art and to provide a novel system and method of compensating for distortion of a received waveform containing digital symbols at one of a predetermined number of phase angles.
It is another object of the present invention to provide a novel system and method for modifying the phase angle of a received symbol as a function of at least the reverse phase shift.
It is yet another object of the present invention to provide a novel system and method for modifying the phase angle of a received symbol as a function of at least the forward phase shift.
It is still another object of the present invention to provide a novel system and method for modifying the phase angle of a received symbol of a continuous phase modulation waveform as a function of the reverse and forward phase shifts for the received symbol.
It is a further object of the present invention to provide a novel system and method for compensating for angular distortion of a received symbol by use of at least one lookup table.
It is yet a further object of the present invention to provide a novel system and method of equalizing a received waveform containing digital symbols at one of a predetermined number of phase angles to reduce the angular distortion of the received symbols.
It is still a further object of the present invention to provide a novel system and method of reducing the intersymbol interference in the form of phase error of a received waveform containing digital symbols at one of a predetermined number of phase angles.
It is an additional object of the present invention to provide a novel system and method of reducing the intersymbol interference in the form of phase error of a received continuous phase modulation waveform containing digital symbols by modifying the phase angle of a received symbol as a function of a predetermined number of reverse and forward phase differences.
It is yet an additional object of the present invention to provide a novel angular distortion compensator.
It is still an additional object of the present invention to provide a novel method of compensating for distortion of a received 16-ary waveform in a satellite communication channel.
It is a further additional object of the present invention to provide a novel non-linear equalizer for a continuous phase modulation waveform.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.